Not trying to be uncooperative here, but I really need a Wiki Pro expert on this. Our site is not a Blog. I can count on one hand how many blogs we have. We are a Wiki.
Also, from my 'points', you can guess that I've been around the block a few times with WP. From my 2 1/2 years using WP I have, many times, turned off all my plugins at the request of WPMU Dev Support, then turned them back on one at a time. Not once has it resolved the problem - not even once.
Additionally, I strive to use as many WPMU Dev plugins as possible. I am sorry to say but many WPMU Dev support folks have recommended plugins from outside sources that are often not tested as compatible with the latest version of WP or haven't been updated for a significant amount of time. I learned the hard way to never install plugins like that.
So let's go back a bit here.
1. By deleting the "private post" media files one at a time, I was able to remove them. Why I couldn't delete a large number at once is odd, but this has now happened at least a few times in the past few weeks, and only the past few weeks. There is something preventing me from deleting more than a few at a time. Any idea? I couldn't find anyone else that has reported this in the Support Forum. Over time I have realized that WIKI Pro is a strange bird on WP and often exhibits problems that are not existent on WP blogs. But the software to delete Media files is part of regular WP, not Wiki Pro. ?????????
2. I update my plugins and themes when they have new versions. Always.
3. How about you, or a Wiki Pro Support tech, check out my site. I will grant permission.
4. As in bullet #1 above, I also checked the support forums regarding the 4700+ entries in my database regarding Membership messages. These records stopped being created in late April last year (2015) - see the screenshot. I know you (WPMU Dev) have had problems since you released Membership. Perhaps someone familiar with that plugin and it's idiosyncrasies over time can give you a better idea why the large number of records were created in my DB by some process that was not initiated by me.
5. User Guest1 has created several dozen wikis. IF I had been able to delete user Guest1, the WP software 'should' have asked me the user name to add all posts created by user Guest1. (of course I have no idea if it just renames the username of the author for regular blog posts, or if also will change the user name of the author of WIKI posts, a 'special' post type. Di you know if it does??)
I have deleted users before, but never was unable to do it. I have even deleted groups of users that were obviously spammers attempting to find a repository for their crap. My setting to approve all posts before publishing stops them. Then I periodically delete the users who attempted to spam us. But this is the first user that I, the super user, couldn't delete. Someone there at WP must know why. ???
6. Last point. Since the 4700+ records stopped being created in April last year, how exactly can I reproduce this problem if it's not happening any more by turning off all the plugins? Can you please ASK someone who is familiar with both WIKI Pro and Membership if there will be ANY problems if I use a database editor and deleted all those records? I backup my database nightly and my entire 250 GB (yes GB) site weekly, so I can recover the DB if there is a problem. But it would be much less time consuming if someone who knows about this problem could tell me, with confidence, that it will or won't cause problems.
I haven't seen your name before when I create a support ticket, so if you are new, welcome to the team. Hope you enjoy "holding our hands" as we navigate the good news and bad news of WP. LOL.
Thanks for your support. I look forward to hearing from you again.
Mary Todd, SysAdmin/Developer/DBA